Devices of this kind are used as nozzles or spray guns in plasma spraying devices. The plasma is produced in the known way, for example, through an electric light arc and a carrying gas. Atomized or powder-form coating materials are introduced into the hot plasma. The resultant plasma jet is directed through a spray jet nozzle onto the workpiece to be coated. Such a nozzle is known from U.S. Pat. No. 3,470,347. Here, a ring-shaped protective gas feed channel is arranged around a spray jet nozzle. This protective gas feed channel is open in the direction of the spray jet and the stream of protective gas is intended to enclose in a ring shape the spray jet lying in the center. Another such device is known from German Disclosure No. 2,818,303. In this device, a protective gas feed channel is also in a ring shape and is arranged concentrically around a spray jet nozzle. However, the outflow direction of the protective gas is directed opposite the flow direction of the spray jet. This leads to flow conditions hard to control between the protective gas and the spray jet.
In the devices described and others for plasma spraying, difficulties occur from time to time since the spray jet coming out of the nozzle is disturbed by various influences. Danger exists in that, through eddying, surrounding air will penetrate into the spray jet. As a result, parts of the coating material will be oxidized. This leads to an unsatisfactory quality of coating. Uncontrolled flow conditions between the protective gas mantle and the spray jet lead to affecting the shape of the spray jet. This may cool off the particles of coating material at the outer portion of the spray jet until it also leads to an adverse effect on the quality of the coating. Since the coating materials are available today, also in powder form, in high purity and in the desired composition, the disturbing influences described, even if they occur only in a slight degree, lead to an undesirable decrease of quality of the coatings.